This invention relates to the field of flow visualization and more particularly to surface flow visualization using heat responsive liquid crystal material for detecting flow separation and transition from laminar to turbulent flow.
There are many methods used today for flow visualization. The method used for a certain situation depends on the type of information desired. Some common methods for flow visualization include tufts, tracers in liquids, surface tracing methods, laser Doppler velocity, thermography, and liquid crystals.
One of the earliest techniques of flow visualization is the use of tufts on a surface. A tuft is a device attached directly to a surface that indicates the flow direction at the surface. The main problem with tufts is that they can disturb the flow characteristics that they are trying to measure. Applying tufts to a model for visualization purposes can increase the drag about the model. Also, tufts have a tendency to exhibit some instabilities, in the form of the end of the tuft whipping around.
Another method for flow visualization is the surface tracing method where visualization is provided by means of a coating on the surface of a body in a flow. The fluid flowing around the body changes the coating, allowing flow characteristics to be determined. The three main methods of surface tracing are chemical, where a reaction between the coating and the fluid occurs, physical, such as sublimation, evaporation, or dissolution, and mechanical, where the change of position or shape of small particles on the surface of the body occurs. One thing to note about surface tracing methods is that the flow visualization pattern found from these methods reflects what is happening at the surface of the model inside the boundary layer, the boundary layer being a thin layer of flow material that adheres to the surface of the body under test. Boundary layer separation occurs when this thin layer of flow material separates from the surface of such body under test and the flow in the boundary layer then becomes reversed. An advantage to surface tracing methods is that the direction of flow can be found during or immediately after the experiment. However, once a surface is coated, it can be used to find results only once and then it must be recoated following a long process of application, drying, solidifying and grinding (in the case of a chemical surface coating).
Another conventional method is laser Doppler velocity which can be employed to determine the local velocity and direction or stream line. Laser Doppler velocity is considered to be non-intrusive but does not always have adequate optical access for complex wall or body shapes and a system might cost on the order of $200K to make these determinations and require several weeks or months to obtain the experimental results.
Thermography is a method used mainly for the determination of the heat transfer coefficient, finding a temperature distribution and sometimes for flow visualization. A thermograph is an instrument used to take instantaneous infrared pictures of a surface. These pictures can be taken over a time period, allowing a continuous time history of the data to be collected and analyzed. Usually, pictures taken using a thermograph go through some form of image processing. Thermography is considered to be non-intrusive and is used mainly in studies of convective flows.
Liquid crystal methods are used to determine heat transfer coefficients and temperature distributions on a surface. Liquid crystals may be described as encapsulated molecules--a state of matter that combines the long-range order of solids with the ability of liquids to form droplets and pour. Thermochromic liquid crystals reflect light at a wavelength which is a function of crystal temperature. If a selected liquid crystal material is applied to a dark background, it will accurately reflect the surface temperature by means of a color distribution. The liquid crystal can be calibrated for a certain color, for example, yellow to reflect at a certain temperature. Liquid crystal methods are considered to be non-intrusive.
A new system for surface flow visualization using liquid crystals is disclosed herein and can be used to determine such flow characteristics as flow direction, separation, reattachment and recirculation. Further, it is non-intrusive and does not physically disturb or distort the flow being measured.